Introduction to Programmable Logic Controllers (PLCs)

Programmable logic controllers
Programmable logic controllers, or PLC's, are used for plant automation and control. The PLC is a specialized, industrial computer which includes onboard random access memory (RAM) and read only memory (ROM). As with any other computer, the PLC has a central processor unit (CPU) for data processing. A single PLC has the switching and logic capability to replace thousands of control relays. PLC's are ubiquitous and are used in many different applications in all industries including semiconductor manufacturing, pharmaceutical production, chemical processing, food production, primary metals, and HVAC. Because of their wide industry use, they are manufactured in many shapes and sizes.

A PLC provides the "I/O" (input/output) which is driven by a control software program. That program interprets, and continuously measures, process inputs and then generates output signals to the PLC's hardware.  Normally, the PLC is programmed for a specific process control application, but they can easily be re-purposed with new programming. As process requirements change, so can the software that runs the PLC.

Small Fixed / Unitary PLC

Small fixed type, or unitary PLC's are smaller, have limited capacity and are used for more basic, simpler control logic - many times on specific-purpose equipment. This type of PLC is used in the controlling of semiconductor processes, compressors, ash handling systems, commercial cooking equipment, pharmaceutical equipment, HVAC equipment, burner control systems, and parts inspection systems. The unitary PLC is small in size, has fewer inputs and outputs (I/Os) and has less memory. Accordingly, they are less expensive. For the most part, these PLC's are used for specific applications. For example, a PLC used for a semiconductor process only monitors the operation of that process. Subsequently, since they are designed for specific purpose, their programming is usually done once and is not changed.

Modular PLC's

Modular PLC's have many inputs and outputs (I/Os) and have their major components broken into "modules" which house the electronics and circuit boards. These modules are designed to slide in to slots in the back plate or back panel, or can be rack mounted in a control panel. The system size and complexity can increased by adding more modules. Modular PLC's are used for more complex process automation for multiple processes in a plant, or even all the processes for the entire plant. Modular PLC's have large data handling capacity and are used for large process control applications. The modular PLC programming is usually customized for the site specifically, and can be re-programmed on-site should requirements change.

Certain types of PLC modules have specific uses and specific functions. Some common main modules are:
  • Power supply modules 
  • Central processing units 
  • Input output modules (I/O) 
  • Ethernet modules 
  • Profibus modules 
  • Redundant communication link
Process Control Solutions' Control Systems group are experts in SCADA and PLC Integration. PCS specialists possess the skills required to program and commission virtually any PLC system including Allen Bradley, GE, Siemens and Modicon. Visit or call (800) 462-5769 for more information.


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Electric Actuators Improve Delayed Coker Operational Uptime and Reliability at Refinery

Reprinted with permission by EIM
EIM 2000 M2CP
EIM 2000 M2CP
Operating ball valves, water purge and slurry gate valves on delayed coking unit process valving.

Refinery in USA

Operating conditions in any refinery’s delayed coker units are extremely severe. Inlet temperatures of the residual oil flowing through the transfer line into the coke drum exceeds 700° F (375°C). During the filling of the coking process, torques are high and tend to increase as the process media builds up in the valve’s wetted parts. The process of coking requires multiple actuators to perform a sequenced series of valve strokes to divert process in a precisely timed event. This sequenced event is controlled by a programmed logic command and must be executed at each actuator reliably and consistently every time. This decoking cycle occurs at regular intervals every twelve to eighteen hours. Upon completion of the cycle, feed is switched to a second drum while the filled vessel is being decoked. During the coke removal process, there is a high level of vibration and water hammer effect as the coke is drilled out by high pressure water jets blasting the coke residue from the lines and coke drums. The introduction of high pressure water into a 700 °F piping system creates rapid expansion and temperature fluctuations. The coke product naturally has traces of sulfur which can be corrosive when subjected to water during the unit wash down. Wash down is necessary due to the abundance of abrasive airborne coke dust that covers every surface within the working coke process unit. The coke dust not only creates challenges for corrosion protection but also builds up in small crevice spaces and impedes instrumentation functionality.

Actuators operating the switching and recirculation valves are subjected to all of these harsh conditions. As such, the Meantime Between Failure (MTBF) of competitors’ actuators in this service has ranged from as little as two months to approximately one year. Valve and/or actuator failure results in unscheduled shutdowns of the coker units and unplanned service repairs or replacement. Actuator failure necessitates manual valve operation and extra manpower. More importantly, process time is extended and, since the coke is used as feed stock in Fluid Catalytic Cracking (FCC) units, production can be curtailed.

Production has increased since installing EIM actuators and maintenance cost has been reduced. The use of EIM 2000 M2CP actuators in this application has been successful due to its robust design and protective coatings. EIM actuators have performed continuously for more than six years in the refinery’s two coker units (eight drums total) where others have failed prematurely. The long life can be attributed, in part, to the Series 2000 gear train durable ductile iron housing and superior powder coating on all surfaces. Because of the rugged design of the EIM M2CP electrical control package and components, the vibration and violent shaking of the actuator during coking process has little adverse effect on the M2CP components. The smaller mass components such as the limit switches are also designed to resist destruction from inertia and momentum generated by operations.The other major design advantage is the ductile iron linear power train Series 2000 gear assembly which is built to withstand the high torque and thrust loads without cracking or deforming. This durability is important in order to maintain precise gear alignment for the rigors of high service demands. The EIM M2CP actuator is well-suited for operations where vibration and water hammer effect are present. There have been nearly no failures or unplanned service requirements in more than six years in the coke application. The plant has been able to conduct shutdowns on schedule. Actuators have been eliminated as a cause of process interruption.

For more information about EIM actuators in Missouri, Kansas, or Illinois, contact Process Controls Solutions by visiting or calling (800) 462-5769.

Control Systems by Process Control Solutions

Process Control Solutions' Control Systems group offers turn-key process automation and systems integration services backed by years of experience in the design and implementation of control systems. Our capabilities include design, specification, fabrication, configuration, programming and start-up of your project. We support a wide range of process applications and requirements including SCADA, PLC & HMI programming, industrial control panels fabrication, product testing and virtually any other custom process automation application. PCS is UL 508 certified and a Wonderware Certified Integrator.

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